Introduction: Hashimoto's thyroiditis disease is an autoimmune disease, occurs in various degrees through various immune mechanisms owing to degradation of thyroid gland. This theory has been proposed that vitamin D deficiency can exacerbate the autoimmune diseases, and its high levels can prevent these diseases. The objective of this study was to compare serum level of vitamin D in patients with autoimmune hypothyroidism and control group subjects. Methodology: In this case-control study, patients with Hashimoto's thyroiditis were referred to the lab to examine the 25-hydroxy vitamin D3 level. For each patient, one healthy subject, who had no exclusion criteria was considered as control and referred to lab for the experiments. Both groups were matched in terms of age, gender, and BMI. Patients' data were recorded in the checklist. SPSS software was used to examine the data. Results: The results revealed no significant difference between Hashimoto's thyroiditis and control group subjects in terms of age, gender, height, weight, 25-hydroxy D3 level (18.05 ± 16.13 versus 16.16 ± 12.67 (p = 0.12), and BMI. Investigations showed no significant convergence between serum level of 25 -hydroxy D and TSH and anti TPO levels. Conclusion: Although there is much evidence on the effectiveness of vitamin D in autoimmunity, including autoimmune thyroid, there is no relationship between serum level of vitamin D and TPO antibody
